This invention relates to liquid crystal devices, and particularly to liquid crystal devices of the type having surface aligned liquid crystal molecules.
One type of recently developed liquid crystal device, see U.S. Pat. No. 3,731,986, issued May 8, 1973 to Fergason (the disclosure thereof being incorporated herein), employs what is known as a "twisted" nematic liquid crystal material. Such devices comprise two closely spaced apart substrates sandwiching a thin film of liquid crystal material therebetween. The inner surfaces of the substrates are treated, as by directional rubbing, to influence and cause alignment of the liquid crystal molecules immediately adjacent to the substrates. In the "twisted" type device, the inner surfaces are so oriented with respect to one another that while adjacent molecules of the liquid crystal material are generally aligned with one another, the direction of the axis of alignment of the molecules gradually rotates or "twists" through some angle, e.g., 90.degree., from one substrate to the other. This angular rotation or "twist" of the axis of alignment of the molecules causes a corresponding angular rotation of the plane of polarization of linearly polarized light directed through the liquid crystal film. Moreover, by selectively applying an electric field through different portions of the film, these portions are "untwisted", with the result that the polarized light passing therethrough is not rotated. Removal of the electric field allows the liquid crystal material to return to its twisted state.
As noted, to obtain the desired alignment of the liquid crystal molecules at the substrate surfaces, it is necessary to "influence" or direct the alignment of the molecules at the substrates. This is done by treating the inside surfaces of the substrates to provide thereon some sort of "grained" microstructure having a definite axis of orientation along which the liquid crystal molecules align themselves. By arranging the microstructure orientation of the two substrates at a preselected angle with respect to one another, the proper twist through the liquid crystal film is obtained. In other types of devices, such as the "guest-host" type described in U.S. Pat. No. 3,551,026, issued on Dec. 29, 1970, the substrate surface orientations are disposed parallel to one another, and no twist is induced through the liquid crystal material.
One means to provide such an oriented microstructure on the substrate surfaces is the use of a process described in an article by John L. Janning, entitled "Thin Film Surface Orientation for Liquid Crystals", Applied Physics Letters, Vol. 21, No. 4, 15 August, 1972, at page 173, the disclosure of said article being incorporated herein. In this process, certain inorganic materials such as aluminum, gold, and silicon monoxide, are vacuum deposited to a thickness of 100A or less onto the substrates using a vapor deposition angle of approximately 5.degree. to the plane of the surfaces to be coated, such process being referred to as "slant evaporation".
While the process described in the article is operative, it is found that certain disadvantages are presented when the process is used as described. For example, the metals disclosed in the article, even when deposited at very small thicknesses, are not transparent, or at least not significantly transparent, and are thus of limited utility in liquid crystal devices of the "transmissive" type, i.e., devices in which it is intended that light pass entirely therethrough with as little attenuation of brightness as possible. While silicon monoxide is transparent, experiments have shown that it is somewhat difficult to uniformly and reproducibly deposit such layers in a simple and inexpensive manner.